122 THE MAMMALIA. 



merly considered identical with it, and described 

 under the name of Equus Stenonis. This matter 

 we shall have to enter upon more fully in a subse- 

 quent chapter. Although not extending farther 

 back than to the Miocene, the period necessary for 

 this reduction of the two toes and of the meta- 

 tarsals to last rudiments (splint bones), was one of 

 enormous length. And yet the modifications that 

 took place during that period in the horse's foot, 

 and the transformation of the dentition that accom- 

 panied the modifications of the foot, were but 

 trifling compared with the differentiations in the 

 group we are at present discussing (the Edentata). 

 There is one other point to consider as regards 

 the approximate length of the period. In the case 

 of the horse, the question was less about a new 

 formation than about parts that lapsed into dis- 

 use ; now such parts are transmitted with in- 

 credible tenacity through long periods of the earth's 

 history. It is evident that more rapidity is 

 shown in transformations where adaptation does 

 not create entirely new organs, but merely modifies 

 those already existing ; thus, for instance, where it 

 changes running feet into climbing feet, and in- 

 significant skin-ossifications into huge shields and 

 plates. 



